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ELECTROCARDIOGRAPHY (II)
MORPHOLOGICAL DESCRIPTION OF THE ELECTROCARDIOGRAM
Scridon Alina, Șerban Răzvan Constantin
1. Propagation of electrical impulses through the heart (Figure 1)
The ECG illustrates the entire electrical activity of the heart, starting with the genesis of the
electrical impulse within the sinoatrial (sinus) node until the depolarization and then the
repolarization of all cardiac myocytes.
Figure 1. The electrical system of the heart.
The primary pacemaker of the heart is the sinus node. However, because of the relatively
small electrical potentials generated by this small cellular mass, depolarization of sinus
nodal cells is not visible on the surface ECG.
After the impulse is generated in the sinus node, it propagates throughout the atria, from
the upper towards the lower regions and from right to left (the sinus node is located in the
upper region of the right atrium, next to the opening of the superior vena cava), causing
atrial depolarization, depicted on the ECG as the P wave.
Then, the impulse is propagated through the atrioventricular node, the His bundle, the two
bundle branches, and the Purkinje network. Within the atrioventricular node conduction is
very slow, while the His-Purkinje system conducts impulses very rapidly. Conduction of
electrical impulses through the atrioventricular node and the His-Purkinje system generates
the PQ (or PR) segment on the surface ECG.
Then, the impulse reaches and activates ventricular myocytes causing ventricular
depolarization, which generates the QRS complex on the surface ECG.
Finally, ventricular myocytes repolarize. Ventricular repolarization takes place in two
steps, first, the slow repolarization, corresponding to the plateau phase of the action
potential, which generates the ST segment on the surface ECG, and then the fast
repolarization, corresponding to phase 3 of the action potential, which generates the T
wave on the surface ECG (Figure 2).
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Atrial repolarization occurs in the same time with and is obscured by the ventricular
depolarization and therefore cannot be seen on the surface ECG.
Figure 2. The elements of a normal electrocardiogram.
2. The electrocardiographic paper
The ECG is recorded on ECG (millimeter) paper. On the ECG paper, the amplitude of the
waves is represented on the y-axis, while the x-axis is the axis of time, where the durations
of ECG elements are measured. If standard settings are used, on the y-axis 1 mV is
represented as 10 mm (1 mm represents 0.1 mV). At a normal paper speed of 25 mm/sec,
on the x-axis 1 sec is represented as 25 mm (1 mm represents 0.04 seconds) (Figure 3).
Figure 3. The electrocardiographic (millimeter) paper.
3. The elements of the electrocardiogram
The morphological analysis describes the elements of the ECG trace recorded during one
cardiac cycle.
The ECG depicts three types of elements: waves, segments, and intervals. The baseline
voltage of the ECG is known as the isoelectric line (Figure 2).
The waves (Figure 4) represent deflections of the ECG trace from the isoelectric line. Every
wave starts at the isoelectric line and ends at the isoelectric line. Waves are described by
their duration (measured in mm, sec, or msec), amplitude (measured in mm or mV), axial
orientation (expressed in degrees) and shape. The waves of the ECG trace are the P, Q, R, S
(forming the QRS complex), T, and U waves, the latter being inconstantly present.
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Figure 4. The normal waves of the electrocardiographic trace.
The segments represent parts of the ECG trace located between two consecutive waves.
Segments are described by their duration (measured in mm, sec, or msec) and position
relative to the isoelectric line. If a given segment is not at the level of the isoelectric line, the
direction (elevation if the segment is above the level of the isoelectric line or depression if
the segment is below the level of the isoelectric line) and the amplitude (measured in mm or
mV) of the change, as well as the shape of the deviation (ascendant, descendant, or
horizontal) must be specified. The segments analyzed on the ECG trace are the PQ (or PR),
ST, and TP segments (Figure 5).
Figure 5. The normal segments of the electrocardiographic trace.
The intervals represent parts of the ECG trace situated between two markers (either the
beginning or the end of certain waves), containing at least one wave. Intervals are only
described by their duration (measured in mm, sec, or msec). The intervals analyzed on the
ECG trace are the PQ (or PR), QT, and RR intervals (Figure 6).
Figure 6. The normal intervals of the electrocardiographic trace.
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3.1. The P wave
The P wave represents atrial depolarization, which starts in the right atrium, within the sinus
node, and is then propagated throughout the atria, from upper regions to lower regions,
and from right to left.
The normal characteristics of the P wave are:
- duration: 0.08-0.10 sec (2.0-2.5 mm), measured at the isoelectric line
- amplitude: should not exceed 0.25-0.30 mV (2.5-3.0 mm)
- axial orientation: between +30° and + 60°. Since the vector of atrial depolarization
points toward the positive pole of lead II and toward the negative pole of lead aVR
(Figure 6), the normal P wave will be positive in lead II and negative in lead aVR.
Figure 6. The vector of atrial depolarization.
- shape: round (See Figure 4), but can be biphasic in lead V1, with a positive component
reflecting right atrial depolarization, followed by a small negative component,
reflecting left atrial depolarization.
3.2. The PQ (PR) segment
The PQ (or PR) segment represents the delay of conduction within the atrioventricular
junction, corresponding with the propagation of the electrical impulse through the
atrioventricular node, the His bundle, the two bundle branches, and the Purkinje network.
On the ECG, the PQ segment starts at the end of the P wave and lasts until the beginning of
the Q (or R) wave (See Figure 5).
The normal characteristics of the PQ segment are:
- duration: 0.02-0.12 sec
- position: isoelectric.
3.3. The PQ (PR) interval
The PQ (or PR) interval represents the interval of time needed for an electrical impulse to be
conducted from the sinus node to the ventricles. Thus, it includes intraatrial, atrioventricular
and His-Purkinje conduction. On the ECG, the PQ interval starts at the beginning of the P
wave and lasts until the beginning of the Q (or R) wave. Thus, the PQ interval includes the P
wave and the PQ segment (See Figure 6).
The normal characteristics of the PQ interval are:
- duration: 0.12-0.21 sec, measured at the isoelectric line.
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3.4. The QRS complex
The QRS complex represents ventricular depolarization and is normally the most ample
element on the ECG trace. The normal characteristics of the QRS complex are (Figure 4):
- duration: less than 0.12 sec, measured at the isoelectric line. The short duration of the
QRS complex reflects the synchronous activation of the right and left ventricles.
- total amplitude: 0.5-1.6 mV (5-16 mm) in the limb leads and the augmented limb leads;
in the precordial leads the amplitude of the QRS complex is larger, because the
ventricles are closer to the exploring electrodes.
- axial orientation: between +30° and + 60°
- shape: polyphasic (contains several, usually three, waves: Q, R, and S), with sharp
waves. When the waves have amplitudes ≥3 mm (≥0.3 mV) they are marked with
uppercase letters (Q, R, S); if their amplitudes are < 3mm (< 0.3 mV) they are marked
with lowercase letters (q, r, s). The three waves are generated by the successive
activation sequences of the two ventricles (Figure 7):
o the first phase is represented by the depolarization of the interventricular septum,
which starts on its left side and propagates to the right. This generates a small r wave
in lead V1 and a small q wave in lead V6.
o the second phase is represented by the simultaneous depolarization of the right and
left ventricles. It is normally dominated by the more massive left ventricle, so that
the vector points leftward and posteriorly. This generates a large S wave in lead V 1
and a large R wave in lead V6.
o the last phase is represented by the depolarization of the posterobasal left
ventricular free wall. The resultant force is directed to the right, upwards, and slightly
backwards. This generates a small s wave in lead V6.
Therefore, ventricular depolarization will generate in a right precordial lead such as V 1 a
small positive deflection (small r wave), followed by a larger negative deflection (large S
wave). In a left precordial lead such as V6 ventricular depolarization will generate a small
negative deflection (small q wave), followed by a tall positive deflection (large R wave), and
a small negative deflection (small s wave). Intermediate leads V3 and V4 show a relative
increase in R wave amplitude (normal R wave progression) and a decrease in S wave
amplitude progressing across the chest from the right to the left. The precordial leads
where the R and S waves are of approximately equal amplitude are referred to as the
transition zone.
Figure 7. The normal aspect of the QRS complex in the precordial leads.
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The first positive wave of the QRS complex is the R wave; if there are additional positive
waves in a QRS complex they are labeled R', R". If the negative deflection between two
adjacent positive apices does not cross the isoelectric line, then the positive deflection is
not considered as two distinct R waves, but a bifid (notched) R wave. The negative wave in
front of the R wave is the Q wave, the negative waves that occur after the first positive
wave are labeled S, S'. If there is no positive wave, the complex being formed by only a
negative wave, this is named QS wave (or QS complex). The QS wave can have a notch on
its descendent or ascendant slope (Figure 8).
Figure 8. The nomenclature of the QRS complex (adapted after
https://commons.wikimedia.org/wiki/File:QRS_nomenclature.svg).
3.5. The ST segment
The ST segment represents the first phase of ventricular repolarization (slow repolarization),
corresponding to the plateau phase of the fast response action potential. On the ECG, the
ST segment starts at the end of the S wave and lasts until the beginning of the T wave (See
Figure 5).
The normal characteristics of the ST segment are:
- duration: has no practical relevance
- position: isoelectric, but small ST segment elevations or depressions are not
considered pathological (≤ 1 mm in V1 and V2 and / or ≤ 2 mm in all other leads).
3.6. The T wave
The T wave represents the final phase of ventricular repolarization (fast repolarization),
during which membrane potential returns to its resting value. Given that the epicardial
region repolarizes before the endocardial region, due to shorter action potential within the
epicardial regions, the corresponding vector is oriented leftwards, downwards, and
forward.
The normal characteristics of the T wave are:
- duration: 0.12-0.30 sec (has no practical relevance)
- amplitude: is evaluated relative to the QRS complex - in the limb leads it should be
approximately 1/3 of the largest R wave
- axial orientation: between +30° and +60°; normally, the T wave is positive in every
lead except aVR
- shape: round and asymmetrical, with a steeper descendant part (See Figure 4).
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3.7. The QT interval
The QT interval represents the entire electrical activity of the ventricles (ventricular electrical
systole) measured form the beginning of the Q wave to the end of the T wave (See Figure 6).
The normal characteristics of the QT interval are:
- duration: depends on the heart rate; considered normal if < 50% of the RR interval.
3.8. The U wave
The U wave is an inconstant, small deflection following the T wave created by ventricular
afterdepolarizations. The significance of this wave remains incompletely understood. It
could result from Purkinje fibers repolarization (Purkinje fibers have longer action
potentials than the rest of the myocardium), or it could represent potentials triggered
during ventricular relaxation (stretch-induced afterdepolarizations).
The normal characteristics of the U wave are:
- duration: 0.15-0.25 sec (has no practical relevance)
- amplitude: less than 2 mm (has no practical relevance)
- shape: round.
3.9. The TP segment
The TP segment represents the electrical diastole. On the ECG, the TP segment starts at the
end of the T wave and ends at the beginning of the following P wave (See Figure 5). This
segment is always isoelectric, and it is therefore used as a reference for the isoelectric line.
The duration of the TP segment is of no relevance, as it varies significantly with changes in
heart rate (becomes shorter in tachycardia and longer in bradycardia).
3.10. The RR interval
The RR interval represents the duration of a cardiac electrical cycle. On the ECG, the RR
intervals lasts from the peak of an R wave until the peak of the following R wave (See Figure
6). The duration of the RR intervals is used to calculate the heart rate (See the
‘Electrocardiography (III)’ chapter).
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TEST YOUR KNOWLEDGE
1.
If the duration of the RR interval is 1 sec, how long is a cardiac cycle?
a. 60 sec
b. 1 min
c. 10 sec
d. 1 sec
e. 15 sec
2.
If the duration of the P wave is 2 mm and the duration of the PQ interval is 5 mm, what
is the duration of the PQ segment?
a. 7 mm
b. 0.3 mV
c. 0.7 sec
d. 120 msec
e. the duration of the PQ segment cannot be calculated
3.
The P wave on the ECG represents:
a. atrioventricular node depolarization
b. sinus node depolarization
c. atrial repolarization
d. atrial depolarization
e. depolarization of the interventricular septum
4.
Which of the following ECG elements represent(s) ventricular repolarization?
a. the TP segment
b. the T wave
c. the QRS complex
d. the ST segment
e. the QT interval
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